An overload relay is an electric switch that is generally used in a field of industrial machines and industrial facilities, as a unit that protects electric devices (such as a motor, hereinafter, “load”) from damage generated due to overheating or an excessive current. This overload relay is generally used as an electromagnetic switch which is combined with a separate relay called an electromagnetic contactor having no current detecting function. This electromagnetic switch is inserted into a current path that connects between a power source and the load, thereby protecting the load to be protected. In other words, when the overload relay detects an overload state, power is supplied to a coil (a trip coil) equipped in the overload relay. A contact part within the overload relay becomes open due to an excitation work of the conducted trip coil, and the excitation of the coil of the electromagnetic contactor that is serially connected to the contact part is cancelled. As a result, the contact of the electromagnetic contactor is operated to become an open state. Consequently, a power supply to the load is interrupted, thereby preventing the load from being damaged.
The overload relay includes a thermomotive overload relay having a bimetal element or a resistive heater, and an electronic overload relay in which a control device such as a microcomputer performs a switch control based on a load current detected by a current detector such as a CT and a hall element. Particularly, the latter electronic overload relay has low power, and therefore, attracts attention as an overload relay with excellent easiness of control. Many overload relays of this type are available.
According to the former typical thermomotive overload relay and the former typical electronic overload relay, when the electromagnetic switch becomes open, a power supply to the own overload relay is also interrupted. Therefore, a recovery operation needs to be manually carried out. A product that drives an overload relay using a separate power source is also available. However, when this product is used, a power source of a separate system needs to be prepared in addition to a power source system that drives the load, which increases the facility cost. Therefore, this product is not used except for a special use.
On the other hand, many industrial machines operate in recent industrial facilities, and their operations are automated. When an operator performs a recovery operation of a trip-operated overload relay at the site at each time of the trip operation, the work efficiency is significantly reduced. Therefore, there has been a request from users for a self-feeding-type overload relay that automatically performs a recovery operation after a lapse of a predetermined time of the trip operation.
For example, Patent Document 1 discloses a technique of preventing execution of a reset operation before cooling of the load or a recovery from an abnormal state, in an overload relay which is not of a self-feeding-type. According to the Patent Document 1, attention is focused on a point that a mechanical thermomotive overload relay has a function of not resetting the operation until when the heater is cooled even when a reset bar operates in a manual reset mode. Based on this, an electronic overload relay realizes a function equivalent to this function of the mechanical thermomotive overload relay.
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-22203